Process for the preparation of trioxane



July 28, 1970 MQNTAUBRIC ET AL I 3,522,278

PROCESS FOR THE PREPARATION OF TRIOXANE Filed July 26, 1966 1261's -12 2 3 r I United States Patent Office Int. 01. (507d 19/00 us. 01. 260-340 1 Claim ABSTRACT OF THE DISCLOSURE The continuous production of trioxane by heating a concentrated aqueous solution of formaldehyde with a strongly acid catalyst under distillation conditions followed by extraction of the resulting trioxane with a water immiscible solvent, here involves the maintenance of a formic acid content in the initial solution of from 3 to by weight, and the use of benzene as the extracting solvent in a ratio with the distillate of from 0.321 to 2:1, the extraction of the trioxane being accompanied by the extraction of a minor amount of formic acid which is separated from the trioxane in the form of an azeotropic mixture with the benzene when the latter is distilled off, and the unextracted but distilled formaldehyde and formic acid is concentrated and recycled in the process.

The invention relates to an improved process for the preparation of trioxane.

1,3,5-trioxane, which is a cyclic trimer of formaldehyde, can of course be obtained by the following steps: heating a concentrated aqueous solution of formaldehyde in the presence of a strongly acid catalyst; continuously distilling 3,522,278 Patented July 28, 1970 catalyst also produces considerable quantities of formic acid which distil out simultaneously with the azeotropic mixture of trioxane and water and the unreacted formaldehyde. The formic acid must therefore be eliminated, since excessive contents thereof would accumulate if it was recycled with the unreacted formaldehyde.

The applicants have found that some of the formic acid in the distillate supplied to the extractor can be extracted at the same time as the trioxane, if benzene is used as the trioxane-extracting solvent. The surprising fact is that if a particular content of formic acid is maintained in the charge of concentrated formaldehyde and strongly acid catalyst fed to the reactor, and a particular ratio between trioxane-extracting solvent and distillate is adapted in the extractor, the trioxane-extracting solvent removes a quantity of formic acid equal to that forming in the reactor.

Advantageously, the formic acid content maintained in the charge fed to the reactor is about 310%.

The ratio between the trioxane-extracting solvent and the distillate adopted in the extractor is advantageously from about 0.3 :1 to 2:1, preferably 1:1.

Benzene is moreover an advantageous trioxane-extracting solvent, since it meets the five following requirements:

(1) It has a boiling temperature lower than that of the water-trioxane azeotropei.e., 9092 C.;

(2) It has an acceptable apportioning coeflicient of the trioxane between the organic solvent phase and the aqueous phase;

(3) It forms an azeotrope with formic acid;

(4) It forms with formic acid an azeotrope containing a considerable quantity of formic acid;

(5) It forms with formic acid an azeotrope whose boiling temperature is lower than that of the water-trioxane azeotrope-Le, 90-92" C.

For purposes of comparison, Table I shows the characteristics of the conventional trioxane-extracting solvents.

TABLE I Apportioning B oiling tem- H C O O H B oiling coefiicient Azeotrope perature of content temperature of trioxane: with azeotrope with in azeo- Solvent 1n C solvent-water HO O OH HO O OH trope Methylene chloride No Ohloroform r. garbon tetrachloride. :::l::: enzene 1. 0. Ti'ichloroethylene 74. l 1,2dichlor0thane 77. 4 14 1 l-dichloroethane 56. 0 5 l-bromopropane. 64. 7 27 2-bromopropane. 56. O 14 l-ehloropropane. 45. 6 8 Pentane 34. 2 10 Cyclohexene 71. 5 69 1,3-eyclohexadiene. 71 70 1,3-hexadiene 71 70 Cyclohexane 70. 7 70 Hexene 71. 5 68. 5 n-Hexane- 60. 6 28 the resulting trioxane in the form of a mixture which is azeotropic with water (boiling between and 92 C.); extracting the trioxane from the distillate by a solvent which is immiscible with water; and separating the trioxane from the extract by distillation.

However, the heating of a concentrated aqueous solution of formaldehyde in the presence of a strongly acid As can be seen from Table I, no conventional trioxaneextracting solvent other than benzene meets the five requirements set forth hereinbefore.

It is also surprising that the apportioning coefficient K when benzene is used as the extracting solvent depends on the quantity of trioxane present in the whole of the two phases and increases therewith.

{ concentration of formic acid in the benzene phase in equilibrium concentration of formic acid in the aqueous phase in equilibrium 3 The trioxane would therefore seem to act as a third solvent with respect to formic acid.

Table H shows results obtained fordifferent quantities of trioxane, the ratio by weight of benzene phase to aque- 4 SOLVENT-SEPARATING STAGE The trioxane is separated by continuous distillation of the formic acid-trioxane benzene extract. An azeotrope composed of formic acid and benzene is obtained at the ous phase being 0.75. 5

TABLE II Initial percent by weight of trioxaue in the aqueous phase 8.8 17.5 26.3 35 43.8 Percent by weight of trioxane in relation to the 2 phases 0 0.013 0.017 0.025 0.033 0.051 0. 065

If the ratio of benzene phase to aqueous phase varies, 15 top of the column. At a place along the column the benthe formic acid apportioning coetficient for the same concentration of trioxane in the initial aqueous phase will also vary, since the trioxane content varies in the whole of the two phases.

Table III shows experimental results obtained with 20 varying ratios.

zene is discharged and recycled to the extractor, and the crude trioxane is discharged at the base of the column. The trioxane is redistilled in a rectifying column. The azeotrope obtained at the top of the column is washed with water until it is neutral. The organic phase is recycled to the extractor.

TABLE III Ratio: benzene phase/aqueous phase 0.25 0.50 0.75 l 1.5 2 Initial percent by weight of trioxane in the aqueous phase 18 18 18 18 18 Percent by weight of trioxane in relation to In general, therefore, the process according to the in- RECONCENTRATING STAGE gg g gg g i s g gs zgg l gf i i z fg i The unreacted formaldehyde remaining in the distillate ex l d g t p g from which the tnoxane has been extracted is reconcen- Stage an a reconcen Ta mg S trated with a make-up quantity of formaldehyde by dis- REACTION AND DISTILLATION STAGE tillation in vacuo. A concentrated formaldehyde is ob- A concentrated aqueous S 01 mi 0 n of formaldehyde is tamed at the foot of the column and returned to the reconstantly introduced into a reactor as distillation is conactor; at the top i column a y dllute solutlon of tinuously performed in the prescnce of a Strong formaldehyde containing methanol is discharged. The soluvolatile acid. The resulting distillate is a mixture of formnon first reconcentrated at pressure to ehmmate the aldehyde trioxane and water containing a small quantity Water and .methanol Whereaftef the preconcemrated form' of methanol and formic acid aldehyde 1s recycled to the inlet of the concentrating The initial formaldehyde solution is an aqueous solution columrll m vacuo of formaldehyde having a concentration of 4065, pref- A P Performamle of the Process .accordmg erably 55 60% of formaldehyde, and 3 10% of formic to the invention is illustrated in the accompanying drawacid.

Strong non-volatile acids, such as sulphuric acid, ben- The coflcentrated f charge fed to a zene sulphonic acid, para to1uenc sulphonic acid, and prep actor 1 via a condult 2. The drstillateleaves the reactor erably Para t01uene sulphonic acid, in a quantity of 1 via a conduit 3, 1s condensed in a C01]. 4 and passes mto 10%, preferably 4 5%, can be used as the catalyst a counter-current extractor 5, from which it is extracted The time spent by the mixture in the reactor can be by benzene Supplied vlaacondlllm- 05-15 hours, preferably 24; hours. The benzene extract passes vla a condu1t7 to a distilling The resulting distillate has the following composition: F f at bottom Whlch P Separated 9 0 is discharged v1a a conduit 9, redistilled in a rectlfying Percent column 10 and recovered via a conduit 11. The formic Tnoxfme 1O 25 acid-benzene azeotrope is discharged from the top of the Formlc acld 0 distilling column 8 via a conduit 12 and is de-acidified by g g a 0 2 being washed with water in a tank 13. The currents of Orma e y e washed benzene arriving from the tank 13 and neutral EXTRACTION STAGE, USING SOLVENT benzene removed from the column 8 via a conduit 12 bis Benzene is used as the solvent to extract the trioxane are recycled the cumin 6 to the.e.xtra.ctor from the resulting distillate which is first condensed at a The unreacted formaldehyde f i m the dlstlnate temperature of C after the extraction of the tnoxane is discharged from the The extraction conaitions are a temperature of about extractor 5 via a conduit 14 and reconcentrated in vacuo 30-50 C., and a ratio of solvent-to-distillate of about ina concentrating column 15 with make'up formaldehyde 03:1 to f bl 11L A Small quantity f the supplied viaaconduit 16. The concentrated formaldehyde formic acid of the distillate passes, encouraged by the 15 returned the Condult 2 to reactor The d1lu te presgnce of trioxane into the solvent phase solution of formaldehyde containing methanol 1s dis- It is a surprising feature that if the formic acid content charged frm the P of concentratfng column and passes via a conduit 17 mto a pressurised preconcentratin the charge fed to the reactor is kept as about 310%, the quantity of formic acid passing into the solvent phase is equal to that forming in the reactor.

Another surprising feature is that such a high content of formic acid can be maintained in the plant Without detriment to the trioxane-forming reaction.

ing column 18. The preconcentrated formaldehyde passes via a conduit -19 into the concentrating column '15. The methanol is discharged at the top of the column 18 via a conduit 20 and the water is discharged at the bottom of the column 18 via a conduit 21.

The invention will now be illustrated by a number of nonlimitative examples.

Example 1 The reactor 1 is fed with a mixture containing 56.9% formaldehyde, formic acid and 38.1% water. After the mixture has spent 2 hours in the reactor 1, in the presence of 5% paratoluene sulphonic acid, a distillate is obtained comprising 39.4% of formaldehyde, 38.1% water, 17.1% trioxane, 5.2% formic acid and 0.2% methanol. The mixture is condensed and extracted with benzene in the extractor 5 in the ratio of 0.72 kg. benzene per kg. of mixture. The resulting benzene extract comprises 80.3% benzene, 19.1% trioxane, 0.2% formic acid and 0.4% water. The extract is distilled in the distilling column 8. At the top of the column -8 the formic acid-benzene azeotrope is discharged, at its centre benzene and water, and at its base crude trioxane. The formic acid-benzene azeotrope is washed with water in the tank 13 and the benzene fraction is added to the benzene discharged at the centre of the column 8. The trioxane is redistilled in a rectifying column to obtain a high degree of purity- The trioxane discharged from the column '10 then has only the following impurities:

Total: methanol, other impurities, etc. 10

Example 2 As in Example 1, the reactor 1 is fed with a mixture containing 56.9% formaldehyde, 40.1% water and 3% formic acid, the result being a distillate comprising 39.5% formaldehyde, 40.1% water, 17.0% trioxane, 3.2% formic acid and 0.2% methanol. The mixture is extracted by benzene at the ratio of 1.5 kg. of benzene per kg. of mixture. The result is a benzene phase formed by 11.4% trioxane, 0.14% formic acid, 0.4% water and 88% benzene. The mixture is treated as set forth an Example 1 to enable a quantity of formic acid to be eliminated equal to that forming in the reactor. After purification, the result is a trioxane of the same quality as set forth in Example 1.

Example 3 As in Example 1, the reactor 1 is fed with a mixture containing 60% formaldehyde, 30% water and 10% formic acid, the result being a distillate formed by 39% formaldehyde, 30% water, trioxane, 10.5% formic acid and 0.50% methanol. The mixture is extracted with benzene, at the ratio of 1 kg. of benzene per kg. of mixture. The result is a benzene phase formed by 16.6% tri- 6 oxane, 0.4% formic acid, 0.4% water and 82.6% benzene. The mixture is treated as stated hereinbefore to enable a quantity of formic acid to be eliminated equal to that forming in the reactor. After purification, the result is a trioxane of the same quality as set forth in Examples 1 and 2.

What we claim is: 1. A continuous process of preparing substantially pure trioxane, which comprises:

heating a concentrated aqueous solution of formaldehyde, containing from 3% to 1 0% by weight of formic acid, in the presence of a strong y acid catalyst, to form trioxane, continuously distilling the resulting aqueous mixture at -92 C., condensing the distillate containing trioxane, 5% to 11% by weight of formic acid, water, unreacted formaldehyde and methanol at 40 C., continuously extracting said aqueous distillate in countercurrent flow with 0.3 to 2.0 parts of benzene per part of distillate, at a temperature of between 30 and 50 C., to obtain a solution of trioxane in benzene containing some water and formic acid, distilling said solution of trioxane to distill off benzene, water and an azeotrope of benzene-formic acid, to leave crude trioxane, distilling said crude trioxane to recover substantially pure trioxane, washing said benzene-formic acid azeotrope with water to separate the benzene and formic acid, discarding the separated formic acid, continuously reusing all of the recovered benzene to extract more aqueous distillate, distilling in vacuo the aqueous solution of formaldehyde, formic acid and methanol resulting from the extraction, to eliminate said methanol and a portion of said water in said aqueous solution, and recycling the resulting concentrated solution of formaldehyde containing formic acid into the trioxameforming reaction together with make-up formaldehyde, whereby the formic acid content in said reaction is maintained substantially constant.

References Cited UNITED STATES PATENTS 3,197,437 7/1965 Wall 260340 3,201,419 8/1965 Sennewald et al 260340 FOREIGN PATENTS 970,142 6/1950 France.

NORMA S. MILESTONE, Primary Examiner 

